Condensors used to condense a vapor have widespread industrial usage and application. Steam condensors in power plants, stills for purifying water or alcohol, and chemical production stills represent a sample of the wide scope in which condensors are needed.
Depending on the relative value of surface tensions between a condensed vapor and a condensing surface, a vapor will tend to condense as a film or as discrete drops. Typically, when the condensate has a higher surface tension than the condensor wall, drop-wise condensation takes place, whereas if the condensor wall has a higher surface tension than the condensate, filmwise condensation occurs.
As an illustration, water has a typical surface tension of 72 dynes per square centimeter. Metals are usually much higher than this, whereas plastics are typically much lower. Thus, if water is condensed on a metal surface, it typically forms a film of water. If it is condensed onto a plastic surface, discrete drops of water are formed instead.
Neugebauer et al in their U.S. Pat. No. 3,206,381 document how that with vertically oriented surfaces film-wise condensation is very effective for low values of heat loading with their implied low temperature differentials across a heat exchanger wall. However, it begins to decrease in effectiveness as the temperature differential and heat loading is increased. On the other hand drop-wise condensation is very effective for large values of heat loading and their implied high temperature differentials. However, it becomes relatively ineffective as the heat loading and temperature differential is decreased. Neugebauer shows that for a typical heat exchanger the crossover point is at about fifteen Fahrenheit degrees difference. When the difference is greater than this, drop-wise condensation is most effective. When the difference is lower, film-wise is better.
Our understanding of the physical phenomenae causing the distinctions in behavior between the two forms of condensation is as follows. Condensate tends to act as an insulator. Thus, the condensation rate is greatest when the quantity of condensate between the condensor wall and the vapor to be condensed is at a minimum.
If condensation takes place as a film onto a vertically oriented surface, then gravity will cause a slow, downward flow of the fluid within the film, causing it to become thinner and thinner. If the rate of condensation is low enough, the film can be kept relatively thin such that a high heat transfer can be maintained at the condensor surface. However, as the temperature difference across the wall of the heat exchanger increases, the rate of condensation also increases. This results in larger and larger quantities of vapor being condensed into the film per unit time. As the condensation rate increases, the downward fluid flow within the film must carry an increasing quantity of condensate. Because of the viscosity of the condensate, the film becomes increasingly thick, which in turn results in a lower heat transfer capability.
On the other hand when the condensation takes place as drops, a different mechanism takes places. The condensor surface is either dry or covered with a drop. When a drop grows in size such that two drops touch, they combine to form a single, larger drop; however that larger drop may actually have a smaller surface area on the condensor than did the two single drops. This is because the condensate within the drops attract each other more strongly than they are attracted by the surface. The dry portions of the condensor surface have a very, very high coefficient of thermal conductivity, far higher than that of a film-wise condensor no matter how slowly it is operating and how thin the film is. As long as the drops remain small enough, the rate of conductivity through the drops is also very high. However, as the drops get larger and larger, the heat transfer rate starts to slow down. With water drops on a polyethylene surface it is our observation that the drops need to reach approximately 1/8 inch in size before they begin to flow of their own accord. Yet, it only takes about one-thousandth of an inch of condensate to begin impacting significantly the overall thermal conductivity.
Once a drop begins to flow, it will flow very rapidly, up to about five feet per second. As it flows down its path, it combines with all the other drops, large and small, it meets and the combination flows together down the surface, again at a rapid rate. The surface is mostly dry after a drop has flowed over it, because the condensate in the drops are attracted to each other more strongly than they are attracted to the surface. One may think of a drop as "washing" a surface clean of condensate as the drop passes by. As long as a steady supply of drops is supplied to a portion of the condensor, that portion will be kept washed of condensate and have a high heat transfer rate. However, if an insufficient supply is provided, then that portion will begin to collect larger and larger drops, and its effective heat transfer is significantly reduced. Basically, the lower portions of a drop-wise condensing surface of a heat exchanger are dependent upon the flow of drops from above to keep them washed clean.
When the temperature difference across the heat exchanger walls is high, it is still possible to have enough heat flowing through the drops to maintain a reasonable rate of vapor condensation into the drops; thus it is possible to generate a steady supply of drops in the upper portion of the heat exchanger which can in turn be used to keep the lower portions washed clean and have their super high transfer rate.
However, when the temperature difference begins to decrease, it takes longer and longer to form drops at the upper surface which are large enough to flow. This results in the lower surfaces not being washed frequently enough to be mostly dry and having only a few drops, with those being quite small. Thus, the size of the drops increases in the lower portion and the overall heat transfer coefficient starts to drop off. With really small temperature differences, such as 1/2 to 1 Fahrenheit degree, the production rate of the falling drops is virtually non-existant. This results in drops forming in the lower portion of the heat exchanger which are too large to transfer heat efficiently but are too small to flow of their own accord. As a result, the overall heat transfer coefficient drops to a useless value.
A condensor which accomodates drop-wise condensation throughout its length may be thought of as divided into two portions, an upper and a lower. The upper portion functions primarily as a drop generator and does not have very high thermal conductivity. The lower functions as the primary heat transfer means and gets washed too frequently to ever generate its own drops.
Plastic condensors have been known in the art for several decades, beginning with Elam in his U.S. Pat. No. 3,161,574, issued in 1963. A plastic condensor can cost less than one percent of an equivalent one made of metal. The typical environment of a condensor is free of ultra-violet light and oxygen; within this environment plastic can outlast metal. Yet, in the commercial marketplace metal, not plastic, is the preferred construction material.
We believe the primary reason for the failure of plastic to function satisfactorily as a condensor is related to its tendency to condense vapors, particularly water vapors, as drops instead of films. In general plastic condensors will need to have low pressure differences across their surfaces. This typically means a low temperature differential across their surfaces. A low temperature differential means low heat loading, and the condensor is attempting to work in that region in which drop-wise condensation is extremely inefficient.